In order to secure data traffic between an external network and a mobile user terminal coupled to the external network, the terminal and a data securing device of the external network produce a pair of private external keys by exchanging partial keys. One item of key information based on at least one of the partial keys, and one message certified by the terminal with a first home location key of the pair of home location keys, are transmitted by the data securing device to the home location network. The certification of the message is verified with a second home location key of the pair of home location keys and a certificate is issued for the key information. The certificate thus provided is transmitted to the data securing device and the pair of private external keys is accepted, subject to the verification of the transmitted certificate, in order to secure data traffic.